Michael Davis, left, and his wife, Desiree, who lost their daughter Claire in a shooting at Arapahoe High School.
Photograph: David Zalubowski/AP

The student who killed a classmate before taking his own life at a suburban Denver high school last year wrote in his diary that he was “a psychopath with a superiority complex”, and he hoped that the attack would start a conversation about school bullying.

Karl Pierson, 18, wrote that he planned the 13 December attack at Arapahoe High School for months to exact revenge for being teased in elementary school, according to investigative reports released Friday.

“Words hurt, can mold a sociopath, and will lead someone a decade later to kill,” he wrote in a document titled “A diary of a madman”.

The victim’s parents said they hoped the shooting would prompt administrators and lawmakers to find ways to improve school safety by focusing on students’ psychological needs.

“The people of Colorado deserve more than to have to sit idly waiting for the next student in imminent crisis to harm or kill another person or themselves,” Michael and Desiree Davis, parents of 17-year-old Claire Davis, wrote in a statement.

Their comments came as authorities offered the first new details about the case since the shooting.

Police have said Pierson held a grudge against his debate coach and was targeting him when he entered the school with a shotgun, a machete, homemade bombs and 125 rounds of ammunition. Pierson shot and killed Davis, who was sitting with a friend in a hallway, before taking his own life in the school library as security officers closed in on him.

The coach, Tracy Murphy, escaped unharmed.

The shootings shocked Littleton Public Schools, one of many across the state that bolstered protocols for identifying the severity of threats and fashioning response plans after the Columbine High School shooting in 1999 left 13 people dead. The two gunmen, both Columbine students, then killed themselves.

School officials determined Pierson was not a danger after a staffer overheard him in a school parking lot threatening to kill Murphy on 3 September, after the coach removed him as captain of the debate team. A school resource officer who is also a sheriff’s deputy took a report of the threat two days later, but the department took no action against Pierson because he hadn’t committed a crime, Sheriff Dave Walcher said.

“That was left in the hands of the school district,” Walcher said. Superintendent Scott Murphy would not say at Friday’s news conference how school administrators concluded Pierson was not a high-level threat, nor would he discuss the school’s handling of the case.

Tracy Murphy, who was so fearful of Pierson he considered resigning, repeatedly asked an assistant principal for surveillance footage that might have captured Pierson yelling the threat, but officials did not search for it until after it had been deleted.

District attorney George Brauchler, who reviewed the report, said he found no criminal offense aside from those committed by Pierson.

Pierson was not suspended and was allowed to return to class less than a week after the threat. His mother had taken him out of school for three days. Officials kept Pierson from attending speech and debate practices, and a school psychologist ruled him a “narcissist” with trouble controlling his impulses, according to the reports.

The threat assessment indicated that the teen would see a psychologist once a week to talk about how to manage his anger. Pierson wrote in his diary in October that a visit with the psychologist was a “massive waste of time” and he was considering going on a shooting rampage at the office where his mother took him for a psychiatric examination. “Let the records show I lied through my teeth through the test.”

Pierson was also suspended in April 2013 for “going off” on a teacher about a poor grade on a math test, his mother, Barbara Pierson, told authorities. She declined to comment Friday night.

Two days before the shooting, Pierson was sent to the assistant principal’s office again after pounding on a locked classroom door and yelling, according to the documents. He was sent home for the day.

As an 18-year-old, Pierson legally purchased his shotgun on 6 December, and he showed pictures of it to friends in the days before the shooting. But no other students were aware of his plans, which he detailed in his diary, Walcher said.

The shooting plot was “a 10-year subconscious project for me to exact revenge, not on the individuals who perpetrated wrong, but instead by those I believe have done me wrong”, Pierson wrote in his diary. “I will do something I have wanted to do for a while – mass murder and be in a place of power where I and I alone are judge, jury and executioner.”

The browser history on Pierson’s laptop included searches for “rampage” and “school massacre”, according to the report. Investigators also found images related to the deadly school shootings at Columbine High School and at Sandy Hook Elementary School in Newtown, Connecticut. Deputies removed the book Columbine: A True Crime Story and a calendar with a countdown to 13 December from the Pierson home.